Abstract: Blocked solvent-free polyurethane (BSFPU) ultrafine synthetic leather base cloth was composed of polyamide 6-alkali soluble polyester (PA6-COPET) sea-island fiber nonwoven and blocked solvent-free polyurethane through impregnation, alkali decrement and finishing method. Microstructures changing during preparation were observed by scanning electron microscope, and then the effect of BSFPU leaching ratio on the mechanical property and hygienic performance of BSFPU ultrafine synthetic leather base cloth was studied. The results show that the sea island fibers are reduced to 37 bundles with a diameter of about 3 μm and the microstructure similar to natural leather is observed in BSFPU ultrafine synthetic leather base cloth. With the BSFPU leaching ratio increasing, the deweighting rate in the process of alkali decrement goes down from 31.59wt% to 28.22wt%; the elongation at break and tear strength of BSFPU ultrafine synthetic leather base cloth reduce continuously; the tensile strength almost remains the same result and the number begins to decline until the leaching ratio above 100wt%; the air permeability also decreases from 50 086 mL/(m²·s) to 24 757 mL/(m²·s) and the water vapor permeability reduces from 790.47 mg/(10 cm²·24 h) to 488.70 mg/(10 cm²·24 h). After testing, BSFPU ultrafine synthetic leather base cloth can meet the industry standards and its hygienic performance is significantly higher than commercially available solvent-based polyurethane ultrafine synthetic leather base cloth.